The hydrophobic associations of three hydrophobically modified alkali swellable emulsion polymers (HASE) were studied by fluorescence. The chromophore pyrene was used as the hydrophobic pendant. The level of association was characterized qualitatively and quantitatively by monitoring the fluorescence behavior of the pyrene monomer and excimer. Excimer formation occurs mostly by diffusion in an organic solvent (tetrahydrofuran, THF) where the pyrene moieties are not associated. In aqueous solutions where pyrene is insoluble, excimer formation occurs mostly via direct excitation of groundstate pyrene aggregates. The level of association of these pyrene-labeled HASEs was determined quantitatively by analyzing the monomer and excimer fluorescence decays using a blob model. It was found that 58% of the hydrophobic pendants were associated in aqueous solutions, but that only 5% of them were associated in THF.